ECE 342 – Jose Schutt-Aine 1

ECE 242Solid-State Devices & Circuits

15. Current Sources

Jose E. Schutt-AineElectrical & Computer Engineering

University of Illinoisjschutt@emlab.uiuc.edu

ECE 342 – Jose Schutt-Aine 2

Amplifier with Diode-Connected Load

Amplifier is single stage M1 driving a load impedance which is that looking into the source of M2

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Output Impedance

22 2 2 2 2

2

ss m s mb s

ds

vi g v g v

r

To calculate output impedance, connect voltage supply at source of M2 and measure current

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Output Impedance

22

2 2 2 2

1ss

s m mb ds

vR

i g g g

1 22 2 2 1

1||out ds S

m mb ds ds

R r Rg g g g

M1 sees this load in parallel with its own output impedance,rds1

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Incremental model

The source to-body voltage changes in M2 use transconductance term gmb2

11

2 2 2 1

mMB m out

m mb ds ds

gA g R

g g g g

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Gain – Diode-Connected Amp

1

2

mMB

m

gA

g

2 ( / )m n ox Dg C W L I

1/ 2

1 1 1 1

2 22 2

2 ( / ) ( / )

( / )2 ( / )n ox D

MB

n ox D

C W L I W LA

W LC W L I

The magnitude of the voltage gain varies as the square root of the aspect ratio

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pMOS Diode-Connected Amp

• Analog Design Requirements – Load device can be

replaced by pMOS transistor– Eliminates body effect of the

load device– Increases the resistance

1/ 2 1/ 2

1 1 1 1

2 2 2 2

/ 3 /

/ /n oxn

MBp oxp

C W L W LA

C W L W L

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Integrated Circuits

• IC Requirements – Biasing of ICs is based on the use of constant

current sources– Use current mirrors– Source circuits are used as loads

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• Analog Design Requirements – Analog ICs may need resistors and capacitors for

the design of amplifiers– Resistors and capacitors occupy the space of

tens or hundreds of MOS devices– It is important to minimize their use

Integrated Circuits

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Transistor Biasing

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Transistor Biasing

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Current Mirrors

A current mirror will reproduce a reference current to the output while allowing the output voltage to assume any value within a specified range. Io=KIin where K is a factor that can be less than or equal or greater than 1

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MOS Current Mirror

2'1

1

1

2D n GS Tn

WI k V V

L

1DD GS

D REF

V VI I

R

2'2

2

1

2o D n GS Tn

WI I k V V

L

R is usually external to IC

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MOS Current Mirror

2 12

1 21

/

/o

REF

W LI W L

I W L W L

Assuming that the transistors are using the same process

• Can be limited by – Channel length modulation (l)– Threshold voltage mismatch– Imperfect geometrical matching

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MOS Current Mirror

2 22 1

1 2 1 1

1

1DS DSPo

REF DS DSP

V VI W L

I W L V V

• Some Properties1. MOS current mirrors draw zero control

currentbetter than BJT’s2. Matching of threshold voltages harder than

in BJT’s

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ExampleA matched pair of MOSFETs are used in a current mirror witl l = 0.032 V-1, mCox=70 mA/V2, W/2L =10, and VT = 0.9 V. Find the value of R to create an input current of 100 mA. Calculate the output current when Vo = 3 V.

2

1 112ox

D GS T DS

C WI V V V

L

Use drain current equation in active region to calculate

2

1 100 700 0.9 1 0.032*D GS GSI V V

We can now solve for the value of VGS

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Example

MOS Current Mirror

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Example

1

1

5 5 1.27237.2

0.1DS

D

VR k

I

The resistance needed is:

The output current is calculated from:

2

2 700 1.272 0.9 1 0.032 3 106DI A

VGS = 1.272 V

2 106DI A

2

2 112ox

D GS T DS

C WI V V V

L

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BJT Current Mirror

2 2

1 1

o s

REF s

I I Area of EBJ of Q

I I Area of EBJ of Q

• Characteristics – Base current is not zero– Depends on relative areas

of emitter-base junction– Want Io=IREF

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Multiple Output Current Mirror